1. Field of the Invention
The present invention generally relates to a casing manufactured of resin by the injection molding process and, more particularly, to such a resin casing having a plurality of cells.
2. Description of the Prior Art
A resin casing having a plurality of cells is required to have high precision and high quality of external appearance. Furthermore, to satisfy these requirements, it is necessary to make the partition wall, defining a cell together with the walls of the resin casing, thinner than that of the side walls of the injection molded casing in order to prevent a shrinkage mark from appearing on the surface of the side walls of the casing.
Hereinafter, an example of the above mentioned conventional resin casing is described with reference to the accompanying drawings.
FIG. 3 shows a perspective view of a conventional resin casing having a plurality of cells. In FIG. 3, reference numeral 11 designates a resin casing, reference numeral 2 a partition wall for dividing the internal space of the resin casing 11, reference numeral 3 a side wall of the resin casing 11, and reference numeral 4 a blister, or raised line, appearing on the side wall 3 of the resin casing 11. FIG. 4 shows a top plan view of the resin casing 11.
According to the resin casing constructed as mentioned above, the shape of the resin casing after injection molding is described hereafter.
In the resin casing produced by an injection molding process, the partition wall 2 generally has a thickness thinner than that of the side wall 3 by 30% to 50% in order to have, after the injection molding, a high surface quality of the side wall 3. This is because if the partition wall 2 had the same thickness as that of side wall 3, cooling will be done unevenly after the molded piece is ejected, resulting in heat accumulation in places at which the cooling takes place after some time delay to cause partial shrinkage. Such a partial shrinkage makes a mark on the surface. Therefore, it is known that the partition wall should be as thin as possible to prevent such uneven cooling.
However, in the resin casing of the above described construction, since the partition wall 2 is so thin, it cools down fast and rapidly becomes stiff. Thus, the apparent shrinkage of the partition wall 2 is smaller than the shrinkage of the bottom plate 20 of the resin casing 11, causing pushing out of the side wall 3 by the partition wall 2. Thus, the side wall 3 is corrugated to form raised lines 4, and thus the outer precision of the resin housing is degraded in the finished product.